Although a high dose of vitamin D3 is known to cause arterial calcification and degeneration, its effect on intimal hyperplasia has never been studied. The aim of this study was to examine the influence of a moderate supplement of vitamin D3 on intimal hyperplasia in the balloon-injured rat carotid artery. Forty-four rats had balloon injury to the carotid artery; 22 were given oral vitamin D3 supplementation (0.25 μg kg−1 day−1). Animals were killed at 4 weeks and the carotid arteries were perfusion fixed and assessed morphometrically by means of computerized image analysis of transverse sections. Mean(s.e.m.) intimal area was significantly greater in the vitamin D3-treated animals than in controls: 0.92(0.05) versus 0.71(0.07) mm2 (P = 0.02). The area of the media of both injured and uninjured arteries was not influenced by vitamin D3 administration. A small dose of vitamin D3 resulted in significant exacerbation of intimal hyperplasia in this rat carotid artery model and raises the question of the role of dietary vitamin D3 in restenosis following vascular intervention.